It has long been known to acoustically log open wellbores to determine the velocities of compression ("P") waves and shear ("S") waves traveling through rock formations located in the wellbore region. Logging devices have been used for this purpose which normally comprise a sound source (i.e., transmitter) and one or more receivers disposed at preselected distances from the sound sources. The use of remotely spaced, multiple receivers is intended to aid in distinguishing between various arriving wave fronts since travel time differentials increase with increasing distance from the transmitter, as described in U.S. Pat. No. 4,383,308 to R. L. Caldwell.
Such arriving wave fronts generally include both headwaves and guided waves. A first arriving event is the headwave commonly called a compressional wave which represents acoustic energy which has been refracted through the formation adjacent the wellbore. This compressional wave travels as a fluid pressure wave in the wellbore mud from the transmitter to the formation where it travels at the compressional wave velocity of the particular formation. The compressional wave then travels to the receiver through the wellbore mud as a fluid pressure wave.
A second arriving event is the headwave commonly called a shear wave which is also refracted through the formation adjacent the wellbore. Unlike the compressional wave, the shear wave travels at shear velocity through the formations. The particles of the formation along the path of propagation are vibrated in a direction perpendicular to the direction of the propagation of the wave.
A third arriving event is the guided wave commonly called a tube wave or Stoneley wave which causes a radial bulging and contraction of the borehole and its travel is, therefore, associated with the borehole wall, that is, the boundary between the borehole fluids and the formation solids.
A fourth arriving event is the guided wave commonly called a normal mode, pseudo-Rayleigh wave, or reflected conical wave. The travel of this normal mode is restricted to the borehole and has an oscillatory pattern normal to its direction of travel. Normally, the shear wave is indistinguishable from the onset of this normal mode due to concurrent arrival times.
Various signal timing and wave front analysis methods have also been suggested for distinguishing between these various wave fronts received at a given receiver. Most of these methods involve timing circuits which anticipate the receipt of, and facilitate the collection of, such wave front information. For descriptions of various logging techniques for collecting and analyzing acoustic wave data, please refer to U.S. Pat. No. 3,333,238 (Caldwell); U.S. Pat. No. 3,362,011 (Zemanek, Jr.); Reissue U.S. Pat. No. 24,446 (Summers); and U.S. Pat. No. 4,383,308 (Caldwell).
In the design of such acoustic logging tools, various types of transmitters, such as piezoelectric or magnetostrictive transmitters, have been suggested for creating the acoustic logging signals. For conventional logging operations, most such transmitters have been centrally located in the borehole, and have been adapted to generate sound which is radiated in a multidirectional (360.degree.) pattern from the transmitter to adjacent wellbore surfaces. Such transmitters are well suited for creating compression waves in surrounding rock and sand formations.
Recently, attention has been directed to developing transmitters which are particularly suited to shear wave logging. Such transmitters generally attempt to achieve a single point force application of sound energy to the borehole wall. The theory behind point force transmitters is that they are capable of directly generating shear waves. Conventional multidirectional transmitters are said to be capable only of indirectly creating shear waves. Point force type transmitters produce shear waves of substantially higher amplitudes than heretofore possible with conventional multidirectional compression wave transmitters. Accordingly, formations such as loosely consolidated or unconsolidated sand, for which shear waves cannot be refracted back into the hole to permit definitive detection using conventional compression wave receivers, may now be shear wave logged with these shear wave logging systems. U.S. Pat. No. 4,049,525 to Angona and Zemanek, Jr. describes a shear wave acoustic logging system employing such a point force transmitter for the shear wave generation.